The present invention relates to a tuneable optical filter, for example, for use in a wavelength division multiplex (WDM) optical communication systems.
Conventional optical communication systems comprise a plurality of spatially distributed nodes interconnected through optical fibre waveguides. Information bearing optical radiation is guided by the waveguides for communicating information between the nodes. Optical radiation in the context of the present invention is defined as electromagnetic radiation having a free-space wavelength substantially in a range of 150 nm to 5 xcexcm.
The information is often modulated onto the optical radiation in a manner of wavelength division multiplexing (WDM), namely the information is subdivided into a number of channels, each channel being modulated onto a corresponding range of optical radiation wavelengths. For example, where 1.5 xcexcm free-space wavelength optical radiation is employed, the wavelength ranges associated with the channels can be sequentially spaced at 0.8 nm intervals. Optical radiation filters are conventionally employed in the systems for isolating radiation associated with specific channels.
When the systems are non-reconfigurable, optical filters therein are set at manufacture to radiation wavelengths of specific channels. However, it is increasingly a requirement that communication systems should be reconfigurable which necessitates such systems including optical filters tuneable over a range of at least several channels.
Although mechanically tuneable optical radiation filters are known, for example in laboratory or astronomical spectrometers, such filters are conventionally regarded as being too costly, unreliable, bulky and slow for use in modern optical communication systems where frequent tuning adjustment is required to select between channels, for example when reconfiguring nodes. Moreover, it is known that precision mechanisms suffer problems of wear when adjusted frequently, such wear giving rise to mechanical backlash which can limit adjustment accuracy; furthermore, such backlash can become worse with prolonged mechanism use. As a result, thermally tuned optical radiation filters and electronically-switchable optical filters are conventionally employed in contemporary optical communication systems.
In U.S. Pat. No. 5,459,799 there is described a tuneable optical filter for use in WDM communication systems. The filter comprises a series arrangement of reflection gratings; each grating is operable to block radiation over a wavelength range of a corresponding channel associated with the grating. Moreover, the gratings are fabricated to block mutually different channels so that the filter is normally operable to block all channels comprising WDM radiation input to the arrangement. An electrode or a heating element is provided for each reflection grating for detuning it; control signals applied to the electrodes or elements can shift the wavelength ranges of their associated gratings to be non-coincident with one or more desired channels to be selectively transmitted through the series arrangement. The arrangement suffers the disadvantage that it is not continuously tuneable; its tuning can only be switched in discrete wavelength steps corresponding to radiation blocking bandwidths of its gratings. Such discrete steps are a limitation if communication systems including such filters are to be upgraded in the future where finer wavelength steps are required, for example where channel wavelength spacings are to be reduced from 0.8 nm to 0.4 nm. Moreover, in order to obtain a fine-tuning resolution, the series arrangement needs to incorporate many reflection gratings which makes the arrangement complex and costly to manufacture.
The inventors have appreciated that it is desirable for optical communication systems to incorporate filters that are continuously tuneable, or at least tuneable in sufficiently fine wavelength steps to cope with future upgrades of the systems. Moreover, in contrast to conventional practice in optical communication system design, the inventors have appreciated that mechanically adjustable optical filters can be adapted to provide acceptable performance in future optical communication systems.
However, conventional mechanisms for precision movement of one or more filter components relative to an optical beam to be filtered through the components tend to be bulky and costly to manufacture; for example, conventional mechanically-adjustable optical filters employ stepper motors and screw drives to actuate one or more optical filter components such as optical gratings relative to an optical beam to be filtered. Moreover, such mechanisms can have limited operating lifetime when subjected to frequent adjustment, such frequent adjustment being manifest in the form of mechanical backlash. Thus, to one ordinarily skilled in the art of optical communication system design, electronically tuneable optical filters are technically preferred to mechanically tuneable optical filters in such systems.
The inventors have appreciated that it is possible to devise an alternative forming of mechanically tuneable optical filter employing an actuating mechanism which relies on step-wise stictional (that is static frictional) movement of one or more filter components relative to a beam of optical radiation to be filtered-through the one or more components.
Thus, according to a first aspect of the present invention, there is provided a tuneable optical filter comprising: moveable filtering means for filtering input radiation to provide corresponding filtered output radiation, the filtering means having spatially varying filtration properties; actuating means for moving the filtering means relative to the input radiation for selecting a portion of the filtering means for use in filtering the input radiation and thereby tuning the filter, characterised in that the actuating means comprises one or more actuating elements operable in a first phase to flex relatively slowly in a driving direction to engage with and maintain full frictional contact with the filtering means to thereby impart a lateral force for moving the filtering means relative to the input radiation in the driving direction, and operable in a second phase to flex relatively more quickly in a direction mutually opposite to the first driving direction such that frictional contact with the filtering means is lost and the filtering means remains in a displaced position.
The invention provides the advantage that it is capable of providing one or more of enhanced adjustment resolution, simpler and lower cost construction and reduced backlash in comparison to tuneable optical filters known in the art.
To provide a simple configuration for the filter, the filtering means preferably comprises a filtering structure having the spatially varying filtration properties and a moveable support member to which the filtering structure is coupled and wherein the one or more actuating elements are operable to repetitively engage with the moveable support member. Such filtering means provides the benefit that the filtering structure can be optimised for filtering the input radiation whereas the support member can be optimised for repetitively engaging the one or more actuating elements. The filtering structure can be formed or supported on a surface of the support member or alternatively be mechanically coupled to the support member.
When the filtering structure is formed or supported on a surface of the support member it is preferable that the support member has a thermal coefficient of expansion selected for compensating for changes in optical tuning characteristics of the filtering structure as a function of temperature. Such compensation assists to ensure that the filter exhibits a more stable tuning characteristic in response to changes in its ambient temperature.
In one embodiment the filtering structure is configured in a partial or complete annulus with the filtering characteristics varying in a substantially circumferential direction and the one or more actuating elements are operable to rotationally actuate the filtering structure relative to the input radiation for tuning. Rotational actuation of the filtering structure enables the filter to be made more compact.
Conveniently, the filtering structure comprises an array of discrete filter regions to provide the spatially varying filtration properties. Such an array provides the benefit that each region can, for example, have filtration properties corresponding to a wavelength division multiplexed (WDM) channel of a communication system into which the filter is accommodated, so that the filter can be tuned on a channel-to-channel basis.
Preferably, the array is a 1-dimensional array providing the benefit that the actuating means only needs to be capable of actuating the filtering means in 1-dimension for tuning the filter and thereby resulting in a relatively simple configuration for the filter.
Alternatively the array can be a 2-dimensional array. The 2-dimensional array provides the potential benefit that the filter can be made more compact compared to using a 1-dimensional array.
It is advantageous that the most frequently selected regions are arranged in relatively close mutual proximity in the filtering means so that the distance the filtering means needs to be moved relative to the input radiation for tuning purposes on average is reduced, thereby improving retuning response time of the filter. Thus, preferably, the array includes most frequently selected regions clustered together in the structure.
Where the regions, for example, the filtration properties correspond to WDM channels in an optical communication system, it is beneficial that the filter provides a fine degree of tuning control for coping with slight wavelength drift within the system. Therefore, it is beneficial that each region has spatially varying filtering properties for use in fine-tuning the filter.
Preferably the regions include one or more of optical gratings and multilayer optical etalons. Beneficially, the regions are operable to at least one of reflect and transmit the input radiation to provide the filtered output radiation; such reflection and transmission provides flexibility in the mechanical design of the filter.
In order to provide a relatively low cost and compact assembly, the moveable support member is preferably a planar member mounted between outer members and the actuating elements are interposed between the outer members and the moveable planar member. In a first such arrangement, the actuating elements are preferably bonded to the outer members and are operable to repetitively engage with the moveable planar support member and thereby impart a, repetitive lateral force thereto for moving the support member relative to the outer members and thereby moving the filtering structure relative to the input radiation for tuning the filter. This first arrangement circumvents a need for flexible electrical connections to the one or more actuating elements that would be necessary if the one or more actuating elements were bonded onto the support member. Alternatively the actuating elements are preferably bonded to the moveable support member and are operable to repetitively engage with the outer members and thereby impart a repetitive lateral force thereto for moving the support member relative to the outer members.
For achieving repetitive engagement of the actuating elements, the elements preferably operable to flex and thereby generate the reprtitive lateral force by employing one or more of piezoelectric, magnetostatic, electromagnetic, electrostatic and thermal expansion effects. Preferably, for reducing cost and achieving compactness, the actuating elements comprise one or more micro-machined components, for example silicon micro-machined components.
For low cost, compactness and relatively low voltage drive requirements, each actuating element preferably comprises a multilayer piezoelectric structure. Moreover, to obtain satisfactory step-wise motion of the filtering means relate to the input radiation, it is desirable that the one or more actuating elements should repetitively engage onto a surface of the support member or outer member with both lateral and normal force components. In order to achieve such engagement, it is preferable that each layer of the piezoelectric structure is polarised in a direction non-normal to major surfaces of the layer. Alternatively, each layer of the piezoelectric structure beneficially has an anisotropic crystalline orientation and is polarised in a substantially normal direction to major surfaces of the layer and is operable to actuate in a non-normal direction to the major surfaces when excited.
In one embodiment the support member comprises a polygonal cross-section bar onto which the one or more actuating elements are operable to impart their associated force. In practice, it is found by the inventors to be particularly beneficial for the bar to be at least one of a substantially triangular cross-section bar and a substantially rectangular cross-section bar.
According to a second aspect of the present invention, there is provided a filter structure for use in a filter according to the first aspect of the invention, the filter structure comprising an array of discrete filter regions for filtering input radiation.
According to a third aspect of the invention, there is provided a method of tuning an optical filter, the filter comprising: filtering means for filtering input radiation to provide corresponding filtered output radiation, the filtering means having spatially varying filtration properties and actuating means for moving the filtering means relative to the input radiation for selecting a portion of the filtering means for use in filtering the input radiation, characterised in that the method comprises: providing in the actuating means one or more actuating elements; and driving said one or more elements with one or more electrical drive signals in a first phase to flex relatively slowly in a driving direction to engage with and maintain full frictional contact with the filtering means to thereby impart a lateral force for moving the filtering means relative to the input radiation in the driving direction, and driving said one or more elements in a second phase to flex relatively more quickly in a direction mutually opposite to the first driving direction such that frictional contact with the filtering means is lost and the filtering means remains in a displaced position.